Abstract [en]

The igneous oceanic crust has long been considered as an inhabitable place on Earth. Research has revealed that deep-sea sediments, and even the igneous crust underneath it, harbours vast quantities of microbial life, and that prokaryotic organisms in the deep- biosphere may contain as much carbon as all plant life found at the Earth’s surface. Detailed studies of the microbiological ecosystem in the deep-biosphere should therefore give us infor- mation about past, present and future environments, since changes in the ocean chemistry, temperatures, topography, and climate have a direct impact on the biotic components, such as diversity, abundance, and morphology.

This study has focused on fossilised microbes found in open and sealed pore spaces in pillow lavas from the Troodos ophiolite in Cyprus. The aim was to understand the geological and geochemical conditions that are needed for microbes to survive under the extreme condi- tions found in the oceanic crust, and was done by investigating the surrounding mineralogy as well as the morphology and chemical composition of fossilized microbes. Quantitative fluid composition, temperature and pH during the time of microbial colonization, as well as fossilisation was evaluated based on the mineralogy and fluid inclusions found in secondary calcite veins.

In this study, it is found that the microbial abundance increases towards higher tempera- tures, more pervasive hydrothermal alteration, and that colonization is favoured in volcanic rocks that are in close association with ore deposits. For their metabolism, they seem to to have preferred colonization around K and Fe rich minerals, vesicle and veins that have had a high abundance of fluids. Fossilization of the microbes has mainly been done by Fe and Mg rich montmorillonite, where some fungi show precipitated, or preserved, goethite in the central strand, and rutile crystals in vacant sites in the clay. Elements such as Mg, Ca, and Na seems to have come in with the oceanic water, and Ti with the primordial water. Fossilization seems to have been initiated during temperature and pH changes of later hydrothermal activity, where at least three hydrothermal events can be seen in total in the samples.

Temperatures <50°C precipitated celadonite and saponite in open vesicles and veins, as well as introduced microbial life into open pore spaces.

A later second hydrothermal event with temperatures <100°C precipitated Na and Ca zeolite, increasing the pH from 4-6 to 7-8, stressing the microbes into starting to adhere clays as possible protection.

Fossilization was finalized with a last hydrothermal event with temperatures >75°C, precipitating Ca carbonates, increasing pH to >8-9, as well as making the environment inhabitable for the microbes.

From this study, it is concluded that microbial colonization in basaltic pillow lavas favours open pore spaces that have had access to a high abundance of fluids, giving rise to more dissolved elements. These elements have come from both the oceanic and primordial water, as well as the host rock, and are essential for the microbes’ metabolism. The microbes seem to prefer temperatures <50°C and a pH below 7-8.